On Wednesday the National Weather Service issued warnings to over 80 million Americans across the Midwest, Northeast, and Appalachian Mountains to limit or avoid outdoor physical activities and exposure. The warnings were triggered by renewed shifts in smoke patterns from the Canadian wildfires affecting Eastern Canada, similar to the exposure experienced by the Northeast a few weeks ago. Just north of the border, Toronto registered the worst air quality of any major city in the world on Wednesday.
Canada is experiencing its worst wildfire season on record, affecting tens of millions of people across both countries. Beyond the obvious immediate dangers and concerns of individuals in the potential path of the fires themselves, the effects on health from exposure to wildfire smoke even at large distances is a growing concern around the world. Affected areas are experiencing ‘unhealthy’ air quality levels that pose risks for at-risk individuals, including children and older adults as well as individuals with medical conditions such as heart or lung diseases.
Effects of wildfire smoke on the brain
The risks and potential health consequences on the heart and lungs are well known. But what is known about the effects of wildfire smoke on the brain and cognition? What are the consequences of acute exposure to potential long terms effects?
The effects of wildfire smoke exposure on the brain even over just a few hours can have significant effects on the health of the brain and its physiology. It can also affect cognition and memory. One of the main culprits is particulate matter (PM) produced by inefficient burning that gets into the air and eventually into the body,
Despite the growing evidence that wildfire smoke PM can have serious effects on the brain, there is no complete picture yet. The number of scientific studies specifically focusing on wildfire smoke exposure on the brain is just a handful, but will likely be an increasing topic of research as the intensity and duration of wildfires continue to increase.
The chemical composition of wildfire smoke PM is different than that of ambient PM or PM from other sources of pollution, e.g. car emissions or industrial pollution. Wildfire PM has a higher proportion of carbon-based pollutants in a variety of chemical forms. The size of PM particles also matters in the effect it has on health. For example, different size particulates can lodge in the lungs differently and have different clinical effects. Wildfire PM consists of a mix of different size particulates. The size of the PM as well as the ecological source from which they’re derived – in other words, where the fire is burning – has an impact on the composition of the smoke, how it travels, and the resultant health effects it produces.
From a cellular and physiological perspective, wildfire smoke is known to be toxic to cells, pro-inflammatory, and capable of affecting how the immune system responds and functions to PM exposure. Particulate matter within size ranges found in wildfire smoke is likely to reach the brain from inhalation via the lungs or the nose (olfactory).
If particulates are entering the brain from the lungs, they need to first cross the blood-brain barrier if they are to access the brain. The blood-brain barrier is a physical barrier formed by what are called tight junctions between the endothelial cells that form the capillaries that carry blood to the brain. Those endothelial cells act as gate-keepers that regulate what can and cannot cross from the systemic circulation and blood into the cerebrospinal fluid that bathes the brain and spinal cord. But the systemic inflammation that results from smoke exposure can increase the permeability – the ‘leakiness’ – of the capillaries that feed the brain, thereby providing a pathway for PM to cross and reach the brain. Inflammation can cause the tight junctions between endothelial cells to become compromised and more permeable, resulting in an unregulated flow of particulates across the barrier.
Wildfire smoke PM could also access the brain more directly through the olfactory neurons in the nose. These neurons extend from the olfactory bulb, which is part of the brain itself, to the olfactory epithelium. As such, if PM enter these neurons and travel to other parts of the brain there is no further need to cross the blood-brain barrier.
Once in the brain, ambient PM have been shown to produce oxidative toxicity that can lead to cell death in neurons and other non-neuronal cells, as well as inflammation. Similar cellular and physiological effects on brain health have been proposed for PM produced by wildfires.
A recent study from 2020 quantified in detail a number of neuroinflammatory and neurometabolomic wildfire smoke inhalation effects in mice, by setting up a mobile lab over 180 miles from active wildfires in California, Arizona, and Washington. Following a four-hour-per-day 20-day exposure, the scientists measured significant and persistent neuroinflammation even as peripheral immune activity in the lungs and bone marrow resolved itself. The researchers were able to show that PM exposure from wildfires directly affected endothelial cells and the expression of a number of specific molecules in the blood and in the brain, as well as an increase in the infiltration of peripheral immune cells into the brain. In the brain itself, they measured an increase in the activation of neural glial cells, which have homeostatic roles that maintain a healthy environment for neurons to function, including molecular changes and activation of microglia, the brain’s resident immune cells.
Interestingly, the scientists also measured a decrease in molecules that normally protect the brain against aging, while simultaneously observing an increase in amyloid-beta protein, which is associated with neurodegenerative disorders such as Alzheimer’s Disease.
Another study documented a statistical connection between short-term acute exposure to wildfire smoke PM with a quantified decrease in a cognitive task consisting of an attention-oriented brain-training game. Exposure to even just a few hours was correlated with decreased cognitive scores. Beyond wildfire smoke exposure, other work has documented the severe mental health effects experiencing a wildfire can have on anxiety, depression, and traumatic stress.
Unknown still, are the potential long-term effects of acute wildfire smoke exposure on the health of the brain and on cognition. And as wildfires continue getting bigger and hotter for longer and longer, the effects of the resultant smoke on the brain of affected individuals could conceivably increase in severity over the decades to come.
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